Electrical feedback detection system for multi-point probes

ABSTRACT

An electrical feedback detection system for detecting electrical contact between a multi-point probe and an electrically conducting material test sample surface. The electrical feedback detection system comprises an electrical detector unit connected to a multitude of electrodes in the multi-point probe, and optionally directly to the test sample surface. The detector unit provides an electrical signal to a multi-point testing apparatus, which can be used to determine if the multi-point probe is in electrical contact with the test sample surface. The detector unit comprises an electrical generator means for generating an electrical signal that is driven through a first multitude of electrodes of the multi-point probe, and a second multitude of switched impedance detection elements. The electrical potential across the impedance detection elements determines the electrical contact to the test sample surface.

The present invention generally relates to an electrical feedbackdetection system for detecting physical contact and/or close proximitybetween a multi-point probe and an locally electrically conducting,semi-conducting or super-conducting material test sample surface andfurther relates to the technique of controlling the relative position ofa multi-point probe and a material test sample surface, and inparticular to an electrical feedback detection system for themulti-point probe and multi-point testing apparatus described inEuropean Patent Application EP 98610023.8 (Petersen), InternationalPatent Application PCT/DK99/00391 (Capres ApS et al), European PatentApplication EP 99932677.0 (Capres ApS), European Patent Application EP99610052.5 (Petersen et al), and International Patent ApplicationPCT/DK00/00513 (Capres Aps et al).

DESCRIPTION OF THE RELATED ART

A scanning tunneling microscope involving controlled approach of asingle tip electrode towards a conducting sample surface is well knownfrom the literature; see for example Binnig and Rohrer, Scanningtunneling microscopy, Helv. Phys. Acta, vol. 55, pg. 355 (1982). Thescanning tunneling microscope consists of a conducting sample and tip,as shown in FIG. 1( a). If the tip and the sample are separated by avery short distance d and a potential V exists between them, a tunnelingcurrentI∝e^(·√φd),is running between the tip and sample, φ being the average work functionof the materials. If the distance d is on the order of 1 nm, adetectable current can be generated. FIG. 1( b) shows a schematic of acomplete scanning tunneling apparatus capable of positioning the tipwithin tunneling distance from the test sample at different testlocations, thereby generating maps of nanometer scale topographic andelectrical features of the test sample.

FIG. 2( a)–(b) shows a schematic of the conventional four-point probe(see for example S. M. Sze, Semiconductor devices—Physics andTechnology, Wiley New York (1985), and published international patentapplication WO 94/11745). The conventional four-point probe consists offour electrodes in an in-line configuration as shown in FIG. 2( a). Byapplying a current to the two peripheral electrodes, a voltage can bemeasured between the inner two electrodes. This allows the electricsheet resistivity of a test sample to be determined through the equationρ=c·(V/I),wherein V is the measured voltage and I is the applied current andwherein c is a geometry factor determined by the electrode separation ofthe four-point probe and the dimensions of the test sample. A principlediagram of the electronic circuit connected to the four-point probe isshown in FIG. 2( b).

FIG. 3( a)–(b) shows a schematic of a conventional microscopicmulti-point probe (se for example published European patent applicationEP 1 085 327 A1). FIG. 3( a) shows the multi-point probe, consisting ofa supporting body and a multitude of conductive probe arms freelyextending from the base of the supporting body. FIG. 3( b) shows amulti-point testing apparatus that implement the mechanical andelectrical means for using the microscopic multi-point probe formeasuring the electric properties of a test sample.

An object of the present invention is to provide a novel electricaldetector mechanism allowing the detection of physical or otherwiseelectrical contact between a multi-point probe and a sample testmaterial surface.

A particular advantage of the present invention is related to the factthat the novel electrical detector mechanism allows the detection ofelectrical connection between a multitude of multi-point probeelectrodes, thereby giving information of the electrical contact of amultitude of electrodes of the multi-point probe.

A particular feature of the present invention is that the novelelectrical detector mechanism does not require a macroscopicallyconducting sample surface, thereby providing detection of electricalcontact to any material surface that contains a local electrical pathbetween several electrodes of the multi-point probe at a specificlocation of the multi-point probe.

The above object, the above advantage and the above feature togetherwith numerous other advantages and features which will be evident fromthe below detailed description of a preferred embodiment of the presentinvention is according to the present invention obtained by a electricalfeedback control system for detecting electrical contact to a specificlocation of a test sample, comprising;

-   -   (a) Electric generator means connected to a first multitude of        electrodes of a multipoint probe;    -   (b) A second multitude of switched impedance detection elements        connecting said first multitude of electrodes of said multpoint        probe; and    -   (c) Electrical detector means for detecting a measuring signal        from the electrical signal across said second multitude of        switched impedance detection elements.

The technique characteristic of the present invention of detectingcontact between a multi-point probe and the test locations of a testsample by utilizing an electrical signal flowing in the multi-pointprobe electrodes avoids the use of laser deflection detection mechanismsin the case of microscopic cantilever based multi-point electrodes,which is a dramatic simplification of the conventional optical feedbackcontrol systems for microscopic cantilever based testing apparatus suchas Atomic Force Microscopes and Scanning Resistance Microscopes.

The electric generator means connected to a first multitude ofmulti-point probe electrodes according to the present invention sends agenerator signal through the test sample at the test location, thatbeing current or voltage, pulsed signal or signals, DC or AC havingsinusoidal, square, triangle signal content or combinations thereof,ranging from LF to HF, in accordance with specific detectionrequirements such as sensitivity to resistance, inductance, capacitanceor combinations thereof, having a LF sinusoidal AC current signal as thepresently preferred embodiment.

The first multitude of electrodes of a multi-point probe according tothe present invention ranges from at least two electrodes to 64electrodes, having the two peripherally posotioned electrodes of themulti-point probe as the present preferred embodiment. Application of agenerator signal to two peripherally positioned electrodes of themulti-point probe provides a resultant detector signal over the secondmultitude of impedance detection elements according to the presentinvention, and infers information about the electrical contactconditions of a third multitude of the multi-point probe electrodes. Anelectrical contact condition can involve physical contact, tunnelingproximity, intermediate fluid meniscus, or any other effect allowingelectrical current to flow between the multi-point probe electrodes andthe test sample.

The second multitude of switched impedance detection elements accordingto the present invention ranges from one to ten, having three as thepresent preferred embodiment. The nominal values of the resistive partof the impedance detection elements ranges from 1 mΩ to 100 GΩ, having 1kΩ, 10 kΩ and 100 kΩ as the presently preferred embodiment.

The electrical detector means measures an electrical signal across thesecond multitude of impedance detection elements according to thepresent invention, having a sensitive electrometer connected to aphase-locked lock-in amplifier as the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and features of the present invention will be morereadily apparent from the following detailed description and appendedclaims taken in conjunction with the drawings, in which:

FIG. 1( a)–(b), provides an overall illustration of the conventionalscanning tunneling microscope. (a), a schematic of the tunneling regionbetween a conducting tip and a test sample. (b), a view schematicallyshowing a conventional scanning tunneling apparatus;

FIG. 2( a)–(b), provides a schematic illustration of the conventionalfour-point probe. (a), shows a schematic of a conventional four-pointprobe in electrical contact with a test sample. (b), shows an electricalschematic of a current source and electrometer connected to aconventional four-point probe;

FIG. 3( a)–(b), shows an overall illustration of the conventionalmulti-point probe and testing apparatus. (a), shows the multi-pointprobe electrodes. (b), is a schematic of the multi-point testingapparatus;

FIG. 4, shows a schematic view of the electrical feedback detectionsystem according to the present invention;

FIG. 5( a)–(b), shows an embodiment of the electrical feedback detectionsystem according to the present invention, in which a multi-point probeis not electrically connected a test sample. (a), shows the detailedelectrical configuration of the electrical feedback detection system.(b), shows the equivalent electrical diagram of the system;

FIG. 6( a)–(b), shows an embodiment of the electrical feedback detectionsystem according to the present invention, in which a multi-point probeis in electrical contact with a test sample. (a), shows the detailedelectrical configuration of the electrical feedback detection system.(b), shows the equivalent electrical diagram of the system;

FIG. 7( a)–(b), shows embodiments of the electrical feedback detectionsystem according to the present invention in which the feedbackdetection system includes a generator of constant electrical current.(a), shows a single switched impedance detection element in the controlcircuit. (b), shows a multitude of switched impedance detection elementsin the control circuit;

FIG. 8, shows an embodiment of the electrical feedback detection systemaccording to the present invention in which the feedback detectionsystem includes a generator of constant electrical current, and thedetector signal is measured between a multitude of the multi-probeelectrodes and the test sample material;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment is directed towards making an electrical feedbackdetection system for a multi-point probe and is described with respectto FIGS. 4–8. FIG. 4 shows a schematic of a multi-point testingapparatus 100 employing an electrical feedback detection system. Theapparatus consists of a multi-point probe 102 in proximity to a testsample 104 which can be moved by a motor stage 108 by means of acontroller 106. The peripherally positioned electrodes of themulti-point probe is connected to an electrical feedback detectionsystem 110, which is capable of determining if the multi-probe 102 is inelectrical contact with the test sample 104. A detector signal 112 isprovided from the electrical feedback detection system 110 to thecontroller 106, to enable a controlled positioning and measurement withthe multi-point probe 102 at test locations on the test sample 104.

FIGS. 5( a)–(b) and 6(a)–(b) together shows the principle of a preferredembodiment of the present invention. FIG. 5( a)–(b) shows a principle ofthe electrical configuration of the electrical feedback detection system300 according to the invention, in a situation where no electricalcontact exists between the multi-point probe 302 and the test sample304. An electrical generator means generates a constant electricalcurrent I_(c), and is connected to the peripheral electrodes 302 a and302 b of the multi-point probe 302. A impedance detection elementconsisting of resistive detection element R is connected to the circuitthrough closed switch SW, and the electrical potential V_(r) across theresistive detection element R is measured by amplifier circuit A Theequivalent electrical diagram of feedback detection system according tothe invention in the situation depicted in FIG. 5( a), is shown in FIG.5( b). The constant current I_(c) runs through the resistive detectionelement R, thereby generating a potential differenceV _(r) =R·I _(c),

Which is measured by amplifier A, and presented at the output of thefeedback detection system. FIG. 6( a)–(b) shows a schematic diagram andequivalent circuit of the feedback detection system 500 in the casewhere the multipoint probe 502 is in electrical contact with the surfaceof the test sample 504. The electrical generator means is connected tothe peripherally positioned electrodes 502 a and 502 b of themulti-point probe 502. A generated current I_(c) flows in part throughthe closed switch SW and the resistive detection element R and thecorresponding electrical potential V_(r) is measured by the amplifiercircuit A, and in part though the test sample 504 represented by unknownresistive element R_(x). In this case the potential difference V_(r) isV _(r)=(R·R _(x))/(R+R _(x))·I _(c).

With reference to FIGS. 5 and 6 it is thus established that theintroduction of electrical contact between a multi-point probe and atest sample generates a well-defined change in the output of thefeedback detection system, hence allowing the detection of changes inthe contact condition of a multi-point probe and a test sample.

In a preferred embodiment of the present invention the constant currentgenerated by an electric generator means I_(c) is 1 μA and the resistivedetection element R has nominal value 100 kΩ, and hence the detectorsignal V_(r) is 10V if no electrical contact is established between themulti-point probe and the test sample. If electrical contact exists tothe test sample, the electrical properties of the test sample give riseto an effective resistance R_(x) of the test sample. The following tableshows the resulting detector signal V_(r) for a range of differenteffective resistance values R_(x) for the test sample:

Relative change in V_(r) from situation of no R_(x) V_(r) electricalcontact  10 Ω 9.99 μV 1,100,000  10 kΩ 9.99 mV 1,100  1 MΩ  909 mV 11100 MΩ 9.09 V 1.1

This shows that the electrical feedback detection system is in thisparticular preferred embodiment of the present invention able to detectcontact to test samples with effective electrical resistances in therange from 10Ω to 100MΩ. In a preferred embodiment of the presentinvention the detector signal is used by the controller of a multi-pointtesting apparatus to determine the electrical contact condition of amulti-point probe to a test location of a test sample, and to activelychange the contact condition by means of electrical signals to a motorstage defining the relative position of the multi-point probe and thetest sample.

FIG. 7( a)–(b) shows detailed implementations of preferred embodimentsof the present invention. In FIG. 7( a), an electrical feedbackdetection system according to the invention 700 has the peripherallypositioned electrodes 702 a and 702 b of a multi-point probe 702connected to a differential voltage to current converter consisting ofamplifier G, resistive detection element R_(set) and voltage followerA1. The resistive detection element R is connected to the output of thevoltage to current converter through switch SW. The output of thevoltage to current converter is proportional to the voltage differenceV₁−V₂. The detector signal V_(r) is measured by means of amplifier A2.The current I_(c) from the voltage to current converter is sent thoughthe closed switch SW and the resistive detection element R and throughthe unknown effective resistance R_(x) in the test sample 704. FIG. 7(b) shows an electrical feedback detection system according to theinvention 800 with multi-point probe 802 connected to test sample 804and electrical feedback detection circuit connected to the peripheralelectrodes 802 a and 802 b of the multi-point probe 802. The electricalfeedback detection circuit contains a multitude of resistive detectionelements R₁ and R₂, which can be individually switched into the signalpath of the electric generator means by means of switch SW, preferableapplication having three said resistive detection elements with nominalvalues in the range 100Ω to 10MΩ.

FIG. 8 shows another preferred embodiment of an electrical feedbackdetection system according to the present invention 1000, withmulti-point probe 1002 connected to test sample 1004 and electricalfeedback detection circuit connected between the peripheral electrodes1002 a and 1002 b of the multi-point probe 1002, and test sample 1004.The generated current l runs in part through the test sample 1004, andthis gives rise to a change in detector signal V_(r) across a resistivedetection element R, even when only one of the multitude of multi-pointprobe electrodes is in electrical contact with the test sample. FIGS. 5a, 6 a, 7 a, 7 b and 8, include filters (301, 501, 701, 801, 1001) forfiltering the output of the amplifier (A, A2).

1. An electrical feedback detection system for detecting electricalcontact of a multi-point probe to a material test sample surfacecomprising: a. electric generator means connected to a first multitudeof electrodes of a multi-point probe; b. a second multitude of switchedimpedance detection elements connecting said first multitude ofelectrodes of said multi-point probe; c. electrical detector meansconnected to the output of the voltage follower for detecting ameasuring signal from the electrical signal across said second multitudeof switched impedance detection elements, and d. an electricalconnection between said electric generator means to said material testsample surface, in which the electric generator means is a differentialvoltage to current converter comprising: e. a precision amplifierproviding two differential inputs, one output, and one reference input;f. a precision resistive element providing an internal and externalport, said internal port connected to said output of said precisionamplifier; and g. the voltage follower providing an input and an output,said input connected to said external port of said precision resistiveelement, and said output connected to said reference input of saidprecision amplifier.
 2. An electrical feedback detection system fordetecting electrical contact of a multi-point probe to an electricallyconducting material surface, comprising: i. electric generator meansconnected to a first multitude of electrodes of a multi-point probe, theelectric generator means being a differential voltage to currentconverter comprising: a. a precision amplifier providing twodifferential inputs, one output, and one reference input; b. a precisionresistive element providing an internal and external port, said internalport connected to said output of said precision amplifier; and c. avoltage follower providing an input and an output, said input connectedto said external port of said precision resistive element, and saidoutput connected to said reference input of said precision amplifier;ii. a second multitude of switched impedance detection elementsconnecting said first multitude of electrodes of said multi-point probe;and iii. electrical detector means connected to the output of thevoltage follower for detecting a measuring signal from the electricalsignal across said second multitude of switched impedance detectionelements.
 3. An electrical feedback detection system for detectingelectrical contact of a multi-point probe to a electrically conductingmaterial test sample surface according to claim 1, further comprising afilter for filtering the output of said electrical detector means,comprising a low-pass filter, high-pass filter, band-pass filter,comparator filter or any combinations thereof.
 4. An electrical feedbackdetection system for detecting electrical contact of a multi-point probeto an electrically conducting material test sample surface according toclaim 1 in which said multi-point probe comprises: a. a supporting bodydefining a first surface; and b. a first multitude of conductive probearms each of said conductive probe arms defining a proximal end and adistal end being positioned in co-planar relationship with said firstsurface of said supporting body, and said conductive probe arms beingconnected to said supporting body at said proximal ends thereof andhaving said distal ends freely extending from said supporting body,giving individually flexible motion to said first multitude ofconductive probe arms.
 5. A multi-point testing apparatus for testingelectric properties on a specific location of a test sample, comprising:a. An electrical feedback detection system according to claim 1; b.means for receiving and supporting said test sample; and c. wherein saidelectric generator means includes means for generating a test signal andelectric measuring means for detecting a measuring signal.